The invention relates generally to wrap-spring clutches and more specifically to a wrap-spring clutch activated by an electromagnet which provides rapid termination of energy throughput.
Electrically and pneumatically powered assembly tools have been utilized for decades on assembly lines. They have greatly assisted productivity where repetitive tightening of fasteners and similar assembly steps are involved. The reduction in worker fatigue, the relative accuracy of torque application and simplicity of operation have all contributed to their acceptance.
As demands for increased production and improved product quality have escalated, however, the operating requirements of such assembly tools have become increasingly sophisticated.
The most common control problem, then and now, of such tools relates to the attainment of proper final torque on a fastener. Too little torque results in an insufficiently secure fastener which may either be obviously loose and require subsequent manual tightening or be not so obviously loose, go unnoticed and loosen in service. Fasteners which have been excessively tightened are a problem of equal magnitude and are equally unacceptable. Over tightening may strain and deform a component or, more commonly, strip the threads and result in a fastener which does little more than occupy the apertures in which it resides.
These and similar torque control problems have resulted in significant effort directed to achieving proper torquing of fasteners. In conventional tools, a variable related to the tool output torque is monitored and when it reaches a desired level, motor power is terminated. In a pneumatic tool, this is generally achieved by closing a valve to shut off air supply to the pneumatic motor whereas in an electric tool a switch is opened to terminate the power flow to the motor. In a pneumatic tool, the monitored variable may be back pressure whereas in an electric tool the variable may be current draw.
For purposes of explanation, if it is assumed that the tool is adjusted to terminate drive energy when the proper torque level is achieved, it will be determined that in both configurations, the inertia of the motor and of any associated speed reduction assembly will cause the tool to overshoot the desired torque level and over tighten the fastener. A rather obvious solution to this problem presents itself: adjust the torque sensor and controller to an artificially low setting so that the torque overshoot of the tool achieves the desired fastener torque level. This solution, itself, creates another difficulty by ignoring another fastener tightening variable.
Fasteners and the components they couple may be characterized as hard or soft depending upon the slope of the torque increase per fastener rotation (or time) occurring while the fastener is tightened. In a hard joint, the torque increases rapidly as the fastener is tightened, a situation encountered when coupling very rigid, noncompressible members. Conversely, a soft joint, that is, one in which the fastener torque increases relatively slowly as the fastener is tightened, is common when one or both of the coupled members are relatively soft and elastic. In the case of a hard joint, setting the torque shut off point at an artificially low level will typically not achieve the desired result because the applied torque will increase with such rapidity that the torque sensor cannot respond rapidly enough to terminate torque application. An over tightened fastener will result. On the other hand, the tightening of a soft joint and the accompanying slow rise in applied fastener torque will result in an under tightened joint due to the anticipatory shut off of the tool. In effect, the solution of lowering the torque shutoff threshold causes the tightening process to be sensitive to the "hardness" of the joint.
Another solution is to simply operate the tool in a relatively slow manner to reduce its motor and drive train momentum and thus torque overshoot. The accompanying increase in assembly time is, however, generally unacceptable.
It is thus apparent that an electrically or pneumatically powered assembly tool wherein the applied torque is monitored, the actual, desired torque value is that value utilized to shut off the tool and the torque overshoot caused by motor and gear train momentum is minimized is desirable. The present invention is directed to just such goals.